Selective emergence of photoluminescence at telecommunication wavelengths from cyclic perfluoroalkylated carbon nanotubes

Chemical functionalisation of semiconducting single-walled carbon nanotubes (SWNTs) can tune their local band gaps to induce near-infrared (NIR) photoluminescence (PL). However, tuning the PL to telecommunication wavelengths (>1300 nm) remains challenging. The selective emergence of NIR PL at the longest emission wavelength of 1320 nm was successfully achieved in (6,5) SWNTs via cyclic perfluoroalkylation. Chiral separation of the functionalised SWNTs showed that this functionalisation was also effective in SWNTs with five different chiral angles. The local band gap modulation mechanism was also studied using density functional theory calculations, which suggested the effects of the addenda and addition positions on the emergence of the longest-wavelength PL. These findings increase our understanding of the functionalised SWNT structure and methods for controlling the local band gap, which will contribute to the development and application of NIR light-emitting materials with widely extended emission and excitation wavelengths.

This manuscript describes the functionalization of single-walled carbon nanotubes (SWNTs) via cyclic perfluoroalkylation. Various 1-iodobutane derivatives were used in conjunction with1,4-diiodo-2,2,3,3tetrafluorobutane and I(CF2)4I to investigate the effects of fluorine atoms and number of reactive sites (one or two) on the functionalization. Absorbance spectra and photoluminescence excitation (PLE) maps were used to characterize the degree of functionalization of the predominantly (6,5) samples through the emergence of two new NIR photoluminescence (PL) peaks, denoted E11* and E11**. Gel chromatography was subsequently performed to separate other chiralities from the sample and investigate the effects of the functionalization. In general, it was found that the degree of functionalization decreased with the number of fluorine atoms and increased with the number of reactive sites; however, the PL wavelength shift of E11* and E11** was larger for the fluorinated molecules. Notably, a highly fluorinated molecule with two reactive sites shifts the PL wavelength to the telecom region of >1300 nm for (6,5) SWNTs.

General Comments and Recommendation
This work builds off previous SWNT functionalization studies by Maeda et. al. It is evident that a lot of effort went into performing this detailed and systematic study, and the general scientific area of the work would be of interest to those in the field of nanotube functionalization and photophysics. While there are some issues/clarifications/inconsistencies that should be addressed, the main concern is the overall nomenclature used throughout the manuscript that renders it challenging to follow. It is recommended that the manuscript be published in Communications Chemistry pending significant editorial modification to present the data and results more clearly.

Editorial comments
-This manuscript provides a systematic study of the effects of fluorine atoms and the number of reactive sites. Therefore, the chemical formulas for the 8 derivatives are all very similar with only a single atom differentiating them. It would be helpful to provide a figure with each of the different substituents, perhaps labeling the part of the molecule that is being systematically changed, and give each molecule a simplified name. For example, the authors published a similar study where they used simplified names rather than the chemical formula to represent the different derivatives (RSC Adv., 2019, 9, 13998-14003). Additionally, Wang et. al. published a systematic study of functionalization where the property being changed (number of fluorine atoms, length of chain) is highlighted (J. Am. Chem. Soc. 2016, 138, 6878−6885). Both methods are effective in helping the reader easily follow the logic behind the study and the results.
-There are several awkward phrases in the manuscript that should be addressed.

Figures
-The figures in the manuscript emphasize the amount of work that went into this study. Because of this, some of the figures are very busy, and in conjunction with the nomenclature described above, can make it challenging to follow. Modifications to the nomenclature would certainly help with this, but additional changes to the figures would also prove beneficial. For example, the matrix in Figure 3 shows the effects of the functionalization on various (n,m) chiralities. Perhaps labeling each column with the respective chirality, and each row with the respective derivative, would help highlight the observed trends. Additionally, in Figure 1, perhaps having the absorbance spectrum with the corresponding PLE map sideby-side and ordered by the systematic change would help highlight the observed trend.
-In Figure S7, each plot has three spectra representing the same derivative. Are these three different trials of the same functionalization?.
3. Discussion -It appears functionalization causes the emergence of the new PL peaks, E11* and/or E11**. However, the reason for why one or both peaks occur with specific derivatives and/or chiralities was not discussed. Why do one (or both) of the peaks emerge as a result of the specific functionalization, and how does that relate to the chirality? -There was discussion regarding why fluorine derivatives are less reactive and why they result in PL at longer wavelengths, but why is having 2 binding sites more efficient and more selective?
-CD was performed on the separated fractions and seem to match results from a previous manuscript by the authors. However, there is no discussion on the relevance of this measurement with respect to this manuscript, and it is therefore unclear how CD adds to the interpretation of the results.
-Is there a reason SWNT-CH2(CF2)2CF3 was not separated using gel chromatography, as the derivatives used for separation were chosen to look at chirlaities other than (6,5) and this derivative has the largest amount of (6,4).

Clarifications/inconsistencies
-Page 6: there seems to be some inconsistencies with the derivatives presented in the manuscript. Specifically, SWNT-(CH2)4 is discussed throughout the results but is not represented in Table S1 nor included in the list of functionalized SWNTs that are being studied.
-Page 6: it should be clarified that "the spectral changes after functionalization tended to decrease with an increasing number of fluorine atom substitutions" was concluded from the D/G ratio.
-Page 10: The term "high PL wavelength selectivity" is somewhat confusing. Do the authors mean the derivative favors longer PL wavelengths, or there is high selectivity in the functionalization? -Page 19: what is meant by "normalized to the measurement time"? I have reviewed "Selective Emergence of Photoluminesecence at Telecommunication Wavelengths from Cyclic Perflouoroalkylated Carbon Nanotubes" by Maeda, et. Al. I believe the results presented are interesting and novel. However, they are iterative on previous efforts to produce SWCNT functionalization that emits in the NIR range. The authors successfully produce a functionalization scheme that generates emission energies of wavelengths longer than 1300 nm. The methods used, however, are a slight variation on previous functionalization schemes that seem to introduce additional electron inducting effects as opposed to generating different configurations or defect isomers. As such, this article is of interest to specialists in the field and has, in my opinion, little general relevance. I think such a report would be well catered for a publication that focuses on nanotechnology or synthetic methods and would not recommend publication in Communications Chemistry.

In this work by Maeda et al., the authors enhanced the optical properties of (6,5)
SWCNTs by using functionalization with perfluoroalkylated moieties. The results are interesting and nicely correspond with the aims and scope of the journal. However, certain issues should be addressed before the paper can be reconsidered for

publication. Please refer to the suggestions given below:
We greatly appreciate your helpful comments. The manuscript has been modified according to your comments.
Revisions are shown with red characters.

1) The SWNT-(CF2)4 notation is misleading as it suggests the grafting of linear (CF2)4 chain onto the SWNT surface. Please consider if SWNT>(CF2)4 is more appropriate. If you agree, the notation on other samples should be changed analogously as well.
We agree with the suggestion. The SWNTs-(CF2)4 and SWNTs-(CH2)4 notations were revised to SWNTs>(CF2)4 and SWNTs>(CH2)4, respectively, throughout the manuscript and supplementary information. Fig. 1a?

How do they correlate with the reported reactivity differences?
We interpret that the characteristic absorption is reduced due to the high functionalization degree in The characteristic absorption peaks decreased significantly by the reaction using 1-bromopropane and 1-bromobutane. Compared to these results, the spectral changes in absorption spectra were suppressed by the reaction using the sterically hindered 2-bromopropane and 2-bromo-2-methylpropane. The significant decrease of absorption peak intensity observed in SWNT-(CH2)3CH3 prepared with 1iodobutane might be due to the higher reactivity of iodoalkane than bromoalkane. The D/G values of SWNT adducts are shown in Supplementary Table 1.

This work builds off previous SWNT functionalization studies by Maeda et. al. It is evident that a lot of effort went into performing this detailed and systematic study, and the general scientific area of the work would be of interest to those in the field of nanotube functionalization and photophysics. While there are some issues/clarifications/inconsistencies that should be addressed, the main concern is the overall nomenclature used throughout the manuscript that renders it challenging to follow. It is recommended that the manuscript be published in Communications
Chemistry pending significant editorial modification to present the data and results more clearly.
We greatly appreciate your helpful comments. The manuscript has been modified according to your comments.

. Additionally, Wang et. al. published a systematic study of functionalization where the property being changed (number of fluorine atoms, length of chain) is highlighted (J. Am. Chem. Soc. 2016, 138, 6878−6885). Both methods are effective in helping the reader easily follow the logic behind the study and the results. -There are several awkward phrases in the manuscript that should be addressed.
According to the reviewer's suggestion, we provide a list of reagents and SWNT adducts highlighted fluoroalkane groups in red, as shown in Fig. 1.   Revised Fig. 1a.

Because of this, some of the figures are very busy, and in conjunction with the nomenclature described above, can make it challenging to follow. Modifications to the nomenclature would certainly help with this, but additional changes to the figures would also prove beneficial. For example, the matrix in Figure 3 shows the effects of the functionalization on various (n,m) chiralities. Perhaps labeling each column with the respective chirality, and each row with the respective derivative, would help highlight the observed trends. Additionally, in Figure 1, perhaps having the absorbance spectrum with the corresponding PLE map side-by-side and ordered by the systematic change would help highlight the observed trend.
The absorption spectra with the corresponding PLE map were displayed side-by-side in Figure 1. The chiral index and notation of SWNT adducts were added in Fig.3.    -In Figure S7, each plot has three spectra representing the same derivative. Are these three different trials of the same functionalization?

Revised
There are the results of three different trials of the same functionalisation. To avoid the readers misleading, Fig. S2, S4, and S7 were deleted.

Discussion -It appears functionalization causes the emergence of the new PL peaks, E11* and/or E11**. However, the reason for why one or both peaks occur with specific derivatives and/or chiralities was not discussed. Why do one (or both) of the peaks emerge as a result of the specific functionalization, and how does that relate to the chirality?
The selective emergence of E11 * PL in the reaction with fluorine substituted iodoalkane was explained by the spin density of reaction intermediates and relative energy of the model compound isomers in the manuscript. The low spin density at the reaction sites and steric repulsion of the fluoroalkyl groups compared to the corresponding alkyl groups suppressed dialkylation, resulting in the formation of hydroalkylated adducts, selectively. The control of the two PL peaks by hydroalkylation and dialkylation was reported previously. SWNT reacted with butyllithium and bromobutane emerged E11 ** PL selectively and SWNT reacted with butyllithium and trifluoroacetic acid emerged E11 * , predominantly. 20 The comparison of experimental results with theoretical calculations of the corresponding model compounds indicated that hydroalkylated SWNT adducts and dialkylated SWNT adducts are plausible candidates for generating the E11 * and E11 ** PL peaks. We revised related sentence as follow.

Revised P.12
'To elucidate the origin of the PL selectivity and significant PL wavelength shifts induced by fluorinated alkyl functionalisation, DFT and TD-DFT calculations were performed systematically ( Figure 5). Previous studies 20,22,26,29 have demonstrated that the relative energy of the functionalised SWNTs and the spin density of the intermediates, which depends on the synthesis route and reagents of the functionalised SWNTs, are relevant factors for the binding configuration. Therefore, these factors, namely, spin density of intermediates, relative stability, and transition energy, of SWNT adducts were focused on in this study' As reviewer pointed out, the two PL peaks were observed in (6,4) and (7,3) SWNT-(CF2)4, and the relation of the chirality is interesting. It is currently unclear why the two peaks are observed in (6,4) and (7,3) SWNT>(CF2)4 even in the theoretical calculations. SWNT>(CH2)4 showed high selectivity in the emergence of new PL peak regardless of chirality. Since the functionalization degree of SWNT>(CF2)4 was lower than that of SWNT>(CH2)4, it is possible that substitution with a fluorine atom reduce the reactivity, decreasing the selectivity of the cycloaddition. On the other hand, the reactivity of small diameter SWNT is known to be higher than that of larger ones. Therefore, the consistent reason for the chirality dependent PL selectivity is not clear. For the clarification, it requires further consideration. The sentence was added as follow.
Revised P. 10 'Interestingly, two PL peaks emerged in (6,4) and (7,3) SWNT>(CF2)4; however, the chirality dependence on the selectivity is currently unclear.' -There was discussion regarding why fluorine derivatives are less reactive and why they result in PL at longer wavelengths, but why is having 2 binding sites more efficient and more selective?
The effect of the tether length of two binding sites has been reported previously, and the results indicate that the alkylation reagents with two reactive sites having the appropriate tether length (n = 3-5) induce new single PL peak in SWNTs. Ring forming reactions using the reagents with two binding sites, I(CF2)4I and ICH2(CF2)2CH2I, can be proceeded in high efficiency and selectively because the reactive sites are more likely to be encounterd intramolecularly. We revised the sentence.

P. 15
'The effect of the tether length of two binding sites in the reactive alkylation using 1,n-dibromoalkane (Br(CH2)nBr has been reported previously, and the results indicate that the alkylation reagents having the appropriate tether length (n = 3-5) induce new single PL peak in SWNTs. 29 It is known that the small ring-forming reaction is stereoelectronically favorable and has high reaction efficiency. The higher PL selectivity observed in the reaction using I(CF2)4I and ICH2(CF2)4CH2I is due to their suitable tether lengths resulting in higher efficiency of cycloaddition reaction.'

-It was mentioned that "Br(CH2)4Br and I(CF2)4I, with two reactive sites, exhibited higher reactivity than CH3(CH2)3I and CF3(CF2)3I." However, there was no further discussion on the bromine comparison.
To clarify the effect of halogen, we conducted the reactions using 1-iodobutane and 1-bromobutane.
When SWNT-(CH2)3CH3 was synthesised with 1-iodobutane and 1-bromobutane, the D/G values were 0.21 and 0.19, respectively, indicating that iodoalkane is relatively higher reactive than corresponding bromoalkene (Table S1). Therefore, we added the sentence in Page 7. The sentence was revised as follow.

P. 7
'Regarding the effect of halogen atoms at the reactive sites, the D/G values of SWNT-(CH2)3CH3 synthesised using 1-iodobutane and 1-bromobutane were 0.21 and 0.19, respectively, indicating that iodoalkane is more reactive than the corresponding bromoalkane.'

-CD was performed on the separated fractions and seem to match results from a previous manuscript by the authors. However, there is no discussion on the relevance of this measurement with respect to this manuscript, and it is therefore unclear how CD adds to the interpretation of the results.
Since the optical resolution of the main component, (6,5) SWNT adducts, was achieved, therefore, the CD spectra was measured and shown in Figure 2. The CD spectra is an important result in understanding the separation behavior of SWNT adducts, therefore, the spectra were shown together with the absorption spectra, as an assignment of the fractions. The sentence was added as follow.

-Is there a reason SWNT-CH2(CF2)2CF3 was not separated using gel chromatography, as the derivatives used for separation were chosen to look at chirlaities other than (6,5) and this derivative has the largest amount of (6,4).
In addition to the selective control of PL wavelength, the lowering the emission energy is main topics in this study. In this point of view, separation of SWNT-CH2(CF2)2CF3 was not rigorously conducted.

Clarifications/inconsistencies -Page 6: there seems to be some inconsistencies with the derivatives presented in the manuscript. Specifically, SWNT-(CH2)4 is discussed throughout the results but is not represented in
The results of SWNTs-(CH2)4 were added in Table S1, Figure S1, and Figure S2.

Revised Table S1
Revised Figure S1 Revised Figure S2 -Page 6: it should be clarified that "the spectral changes after functionalization tended to decrease with an increasing number of fluorine atom substitutions" was concluded from the D/G ratio.
As pointed out by reviewer, the tendency was considered from D/G values. The sentence was revised as follow.  Table 1).' Figure 4 and Table 1

, which shows the results of the separated SWNTs, SWNT-(CH2)4 and SWNT-CH3(CH2)3-SWNT-H (derivatives not listed above) are both shown, but SWNT-CH2(CF2)2CH2 is not. The derivatives shown in the figures/tables should be consistent with what is described in the manuscript.
Thank you for your precise remarks. The data and plots for SWNT>CH2(CF2)2CH2 were added to the